There are known photoelectric pyrometers for measuring temperatures of stationary and moving elongated bodies of small cross-sectional dimensions (cf. A. I. Gordov, "Osnovy pyrometrii"/"Principles of Pyrometry"/, metallurgia Pulblishers, Moscow, 1964, pp. 352-353).
In such pyrometers, the light-sensitive surface of a photoelectric converter, arranged in the sharp-image plane of an objective, receives one (direct amplification) or two (comparison with optical feedback) trains of light pulses. In the former case, the single train of light pulses is produced as a luminous flux passes from a heated body through an objective with a diaphragm, an obturator, and a condenser with a diaphragm. In the latter case (comparison with optical feedback), the second train of light pulses is produced by the same obturator arranged across the path of a luminous flux generated by an optical feedback radiation source.
The photoelectric converter produces trains of electric pulses shaped as a distorted sinusoid; these pulses are applied to an amplifier; the amplification may be either direct or determined by the results of a comparison with optical feedback data. In order to use pyrometers of the type under review for measuring temperatures of vibrating bodies of small cross-sectional dimensions, the geometrical dimensions of the light-sensitive layer of the photoelectric converter must be several times less than those of the image of the body whose temperature is measured; as a result, one must use photoelectric converters with light-sensitive surfaces of a small area, as well as long focal length telephoto lenses.
There is further known a photoelectric pyrometer for measuring temperatures of bodies of small cross-sectional dimensions which vary with respect to the optical axis of the pyrometer. The photosensitive surface of this type of pyrometer receives two trains of light pulses. The first train of pulses is produced as a radiation flux passes from a heated body through an objective with a stationary field diaphragm, a slit obturator, and a condenser with an aperture diaphragm. The second train of pulses is produced by the same obturator arranged across the path of a luminous flux generated by an optical feedback radiation source. The two trains of light pulses are converted into two trains of initial electric pulses applied to an amplifier which is electrically coupled to a storage and comparison device electrically connected, in turn, to the optical feedback radiation source and a secondary recording instrument (cf. USRR Inventor's Certificate No. 185,513, Cl. G 01 j 5/28).
In this type of pyrometer, the luminous flux passes from the heated body through the objective with the stationary diaphragm and is focused in the sharp image plane of the objective. This plane is tangential to the cylindrical surface of a rotatable drum having a narrow slit parallel with the rotation axis and the axis of the elongated body whose temperature is measured; the drum serves as an obturator. Arranged inside the drum, along its radius, are a light filter, a condenser with an aperture diaphragm, and a right-angle prism oriented so that the image of the slit, which is illuminated as it traverses the image of the heated body in the sharp image plane of the objective, is received by the light-sensitive surface of a photoelectric converter arranged outside the drum and coaxially with it. Thus the condenser trasmits, on a certain scale, the image of the illuminated rotating slit to the light-sensitive surface of the photoelectric converter, and the image traverses that light-sensitive surface.
The photoelectric converter converts the light pulses to a first train of initial electric pulses whose duration is determined by the magnification ratio of the objective-condenser optical system and the cross-sectional dimensions of the heated body.
As the drum turns through an angle of 180.degree., the slit traverses the image of the optical feedback radiation source placed in series with a rheochord of the comparison circuit, which image is focused by the second objective in its sharp image plane tangential to the cylindrical surface of the drum. The condenser and prism rotate together with the drum, so the image of the illuminated rotating slit, whose brightness is dependent upon the brightness of the optical feedback radiation source, is also transmitted to the light-sensitive surface of the photoelectric converter which produces a second train of initial electric pulses. The pulses of the second train are found between those of the first train.
The two trains of initial electric pulses formed by the photoelectric converter are applied to the amplifier having a feedback input, and to a converter which is, in fact, an analog storage and comparison device. The converter generates voltage which is proportional to the difference between the amplitudes of pulses of the two pulse trains. The voltage is applied to a power and voltage amplifier having two outputs of which the first is intended to transmit feedback voltage to the first amplifier, whereas the second is intended to switch on the rheochord's drive motor connected in series with a feedback lamp and a calibrated resistor. The amplifier supplies voltage to the drive motor of the rheochord until the pulse amplitudes of both trains of pulses produced by the photoelectric converter become equal.
The supply current of the optical feedback radiation source is unabbiguously identified with the temperature. The voltage drop across the calibrated resistor is measured by a secondary recording instrument which is an electronic potentiometer whose scale is graduated in degrees Celsius.
As pointed out above, the motion of the heated body with respect to the optical axis of the pyrometer is accompanied by movement of the image of the illuminated slit across the light-sensitive surface of the photoelectric converter. This accounts for measurement errors which are due to the fact that the light sensitivity of the light-sensitive surface and the rate of its ageing with time are not uniform.
If there is a change in the temperature of heated bodies of different cross-sectional dimensions, the duration of the initial electric pulses of the pulse train produced in the course of scanning of the light flux coming from the heated body is proportional to the cross-sectional dimensions of the heated body; the initial electric pulses of the pulse train produced in the course of scanning of the light flux coming from the optical feedback radiation source are of a constant duration.
As a result, there is an extra measurement error dependent upon the size of the heated body and the distance to that body. The reasons for this are as follows:
No matter what specific design is chosen for the analog storage and comparison device, this device necessarily comprises a storage capacitor; the potential at the plates of this capacitor increases exponentially with time and at infinity reaches the amplitude of the pulse applied to the capacitor; an acceptable degree of accuracy is practically ensured within a period of time of no less than 4.div.5 time constants of the charging circuit (with due regard for the internal resistance of the source).
The initial electric pulses are trapezoidal pulses. It must be remembered in this connection that the distance covered by a pulse curve along the x-axis is proportional to the energy of the pulse, and that the ratios between the energies of the leading and trailing edges and the total energies of pulses of different durations and equal amplitudes are different; hence, the charge levels of the capacitor of the analog device are different in such cases.
In addition, it is hard to manufacture and balance a rotatable drum with a slit, while the electronic circuit, intended to adjust the radiation source's current with the use of the electromotor-rheochord system, features a great time constant, whereby the sphere of application of the pyrometer under review is quite limited.